We have now confirmed that this issue was caused by an update to the Chromecast firmware that is being rolled out to devices. It appears that Android TV devices received it first, however some Chromecast Ultra devices also have it.
The issue currently only affected users who used an iOS device to start the stream - Android users were unaffected.
A patch has been applied to the Chromecast receiver - if you're already connected to your Chromecast then disconnect and reconnect and it should now work fine.
